A new hyperrecombinogenic mutant of Nicotiana tabacum.

We have isolated a hyperrecombinogenic Nicotiana tabacum mutant. The mutation, Hyrec, is dominant and segregates in a Mendelian fashion. In the mutant, the level of mitotic recombination between homologous chromosomes is increased by more than three orders of magnitude. Recombination between extrachromosomal substrates is increased six- to ninefold, and intrachromosomal recombination is not affected. Hyrec plants were found to perform non-homologous end joining as efficiently as the wild type, ruling out the possibility that the increase in homologous recombination is due to a defect in end joining. In addition, Hyrec plants show significant resistance to gamma-irradiation, whereas UV resistance is not different from the wild type. This suggests that homologous recombination can be strongly up-regulated in plants. Moreover, Hyrec constitutes a novel type of mutation: no similar mutant was reported in plants and hyperrecombinogenic mutants from other organisms usually show sensitivity to DNA damaging agents. We discuss the insight that this mutant provides into understanding the mechanisms of recombination plus the potential application for gene targeting in plants.